[TRAVIS CI] fails due to ppa configuration issues

[plbossart]

not sure what's going on:

https://travis-ci.org/github/thesofproject/linux/jobs/721742486
https://travis-ci.org/github/thesofproject/linux/jobs/721742488

The ppa configuration seems wrong?

@marc-hb @xiulipan FYI

[marc-hb]

Well, from https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test

You can update your system with unsupported packages from this untrusted PPA by adding ppa:ubuntu-toolchain-r/test to your system's Software Sources.

We're just getting what we paid for?

This repo was added years ago by @xiulipan in commit 9429ffc, to upgrade (!) gcc to gcc version 7 so it could compile kernel 4.20.

As you can see from the top of the Travis log, Ubuntu 18.04 already has gcc 7.5 by default so I suggest reverting 9429ffc. It will make the build not just simpler but faster.

[xiulipan]

@marc-hb Agreed, we may need some update on our Travis scripts. Now maybe Travis can support Ubuntu 20.04 and the default GCC can satisfy the Kernel build require. I will send a update patch.

[xiulipan]

@plbossart @marc-hb As the Travis now has Ubuntu 20.04 support with GCC 9.3 https://docs.travis-ci.com/user/reference/focal/ I will directly remove the PPA thing and use Ubuntu 20.04 with GCC 9.3 now.

[xiulipan]

PR send #2402